1. Field of the Invention
The present invention relates to a method and apparatus for registration of multiple scans of internal properties of a body, such as internal tissues of a living body; and, in particular to a method and apparatus that implement techniques to accelerate determination of an array of spatial transformations that elastically register an early scan to a current scan in time for a particular use, such as in time to adjust a planned treatment of the body to current conditions of the internal properties based on the current scan, or to efficiently produce an atlas or composite or diagnosis from a large number of scans.
2. Description of the Related Art
Healthcare delivery, whether financed by private or public funds, amounts to a multi-billion dollar business. Advances in healthcare improve the product obtained for those dollars and renders older techniques obsolete.
Several modern techniques for treatment of diseases of internal tissues of living bodies involve directing treatment to particular tissues and avoiding others. For example, in radiation therapy, one or more radioactive sources or beams of high energy particles are placed or focused on diseased tissues, such as tumors, while avoiding healthy tissues. In invasive radiology, probes are inserted into a living body to extract diseased tissue or introduce therapeutic agents at the site of particular diseased or healthy tissue in a living body. In laparoscopy, surgical instruments at a tip of a probe excise tissue and implant fasteners and other devices. The efficacy and safety of such directed treatments are affected by the accuracy of placement of the treatment.
Several technologies are available for non-invasively measuring the arrangement of tissues within a living body. These technologies, often called imaging technologies, produce scans of spatially arranged scan elements that depict spatial variations in measured quantities that are related to spatial changes of one or more physical properties in the tissues. Scan elements can be presented as images in color or grayscale intensities. Often the measured quantity is intensity of electromagnetic or acoustic energy received in some time interval from some direction. The measured quantity depends on the spatial arrangement of absorption or speed in the intervening tissues, which in turn varies with the type of tissue. Well known imaging technologies includes computer-aided tomography of low intensity X-rays (CT) at regular doses and at low doses, nuclear magnetic resonance (NMR) imaging (MRI), positron emission tomography (PET) and ultrasound (US) imaging, among others. In various arrangements, two-dimensional (2D) and three-dimensional (3D) scans are formed. Such scans are also called images. Scan elements in a 2D scan are sometimes called picture elements (pixels) and scan elements in a 3D scan are sometimes called volume elements (voxels).
In stationary tissues, such as those within the skull when the skull is intact, the spatial arrangement of the tissue is constant and well known by fixing the position of certain external skeletal features that are used as landmarks, and collecting one or more images relative to those landmarks.
However, soft tissues outside the skull are able to flex and change size, shape or position over time, even when referenced to certain skeletal features that can be fixed. For example, organs and tumors in or near the thoracic cavity ebb and flow with the breathing of the living body. Tissue in and near the heart move with the beating of the heart. Tissues near the gastrointestinal track and urinary bladder, including the bladder and prostate in the human male, swell and shrink with the amount of consumed food and fluids being processed by the living body, and by the history of physical movement of the living body between scans. In brain surgeries performed open or endoscopically, the soft tissue motion and deformation problem occurs when the surgeon starts removing the tumor tissue.
For directed treatments that are administered over times long compared to the time scales of such flexing of soft tissue, a single scan of the soft tissue, no matter how high the spatial resolution, is not accurate for the entire treatment. Thus radiation directed to a target tissue (e.g., cancerous prostate) based on a single CT scan of the prostate can lead to irradiating non-target tissue during part of the treatment time, and failing to irradiate some target tissue during part of the treatment time. Similarly, navigating a probe according to a plan based on a planning image taken on one day may lead the probe incorrectly on a different day when treatment is administered.
In some past approaches, the treatments are based on one or more scans at a single time and the treatment area is expanded to treat all positions through which the target tissues may move during the treatment, e.g., expanding the treatment area beyond the target area by some amount or percentage that is expected to cover normal flexing of the soft tissue. While suitable for some applications, this approach suffers from the disadvantage that some non-target tissue is exposed to the treatment. For example, some healthy bladder and rectal tissue is subjected to radiation intended to kill cancerous prostate tissue.
In another approach, multiple scans are taken at different times and different treatments are applied for different scans. A problem with this approach is that some scans, such as CT scans, take many minutes to perform, are expensive, expose a patient to hazardous radiation, and can obstruct access to the tissue by the treatment provider, such as an interventional radiologist or a therapeutic radiation source. In many cases, such scans are not appropriate through the course of the treatment (a time period called herein “intra-treatment”). Another problem with this approach is that an analysis or treatment plan for the new scan may involve human interaction and can take considerable time to develop.
In some cases, one or more intra-treatment scans are taken using technologies that are faster, cheaper, safer or less obstructive, such as ultrasound which enjoys all four advantages. Other technologies appropriate for intra-treatment measurements include low-dose CT scans. It is anticipated that other appropriate imaging technologies will be developed in the future. However, a problem with this approach is that the intra-treatment scan technology often does not provide the spatial resolution needed. For example, in ultrasound there is low contrast between the prostate and bladder tissue compared to CT scans, and there is more noise in the form of speckle. Similarly, low dose CT has a lower signal to noise ratio. This approach also suffers the disadvantage that an analysis or treatment plan for the new scan may involve human interaction and can take considerable time to develop.
In such cases it would be beneficial to register the intra-treatment scan to a treatment planning scan taken and analyzed before treatment begins. Even if intra-treatment scans provided sufficient resolution, doctors would continue to use pre-treatment scans for planning and rehearsal. Thus there would still be a need to register pre-treatment scans associated with treatment plans to intra-treatment scans. Image registration is the process of aligning two or more images that represent the same object, where the images may be taken from different viewpoints or with different sensors or at different times or some combination. A transformation that aligns two images can be classified as rigid, affine, or elastic (including projective and curved transformations). Rigid transformations include translation or rotation or both. Affine transformations add shear or scale changes or both. An elastic transformation is a special case of a non-rigid transformation that allows for local adaptivity (e.g., uses a transform that varies with position within the scan) and is typically constrained to be continuous and smooth.
Because soft tissue can change shape and size as well as position, an elastic transformation is desired (provided by a so-called elastic registration process), rather than a rigid body or global affine transformation. A global affine transformation is an affine transformation that is applied to an image as a whole.
Thus, it would be beneficial if an analysis or treatment plan developed for an original scan (the treatment planning scan) can be elastically transformed to the new measurements by elastically registering the original scan to the new scan to account for the soft tissue flexing that has occurred since the original scan.
To be useful for treatment, however, the elastic registration (called intra-treatment elastic registration, herein) must be determined on a time scale short compared to the treatment duration. A problem with many current implementations of elastic registration is that they involve more computations and are more time consuming than rigid body or global affine transformations. Indeed, some experts have taught that elastic registration is rendered inappropriate for rapid intra-treatment applications (see, for example, W. R. Crum, T. Hartkens, and D. L. G. Hill, “Non-rigid image registration: theory and practice,” The British Journal of Radiology, vol. 77, pp. S140-S153, 2004).
A recent approach has been presented to provide hardware acceleration for a particular type of elastic registration, one based on the computation of mutual information (MI) as a measure of similarity that is maximized to find the best registration. See C. R. Castro-Pareja, J. M. Jagadeesh, R. Shekhar, “FAIR: A hardware architecture for real-time 3_D image registration,” International Electronics and Electrical Engineers (IEEE) Transactions on Information Technology in Biomedicine, vol. 7, no. 4, pp. 426-434, 2003 (hereinafter Castro-Pareja I) and C. R. Castro-Pareja, R. Shekhar, “Hardware acceleration of mutual information-based 3D image registration,” Journal of Imaging Science & Technology, vol. 49, no. 2, pp. 105-113, 2005 (hereinafter Castro-Pareja II), the entire contents of each of which are hereby incorporated by reference as if fully set forth herein. While able to accelerate the computations of elastic registration to suitable speeds for some applications, there is need for further speed improvements for some kinds of scans and for some intra-treatment time scales, such as treatment of tumors in the lung or the treatment of sources of arrhythmia in the heart or determining when to activate a probe tip during interventional radiology.
Based on the foregoing, there is clear need for techniques based on one or more intra-treatment scans that provide intra-treatment elastic transformations of an analysis or treatment-plan from a treatment-planning scan, which do not suffer one or more disadvantages of prior art approaches.
In particular, there is a need for intra-treatment elastic registration between multiple scans of soft tissue in a living body, which do not suffer one or more disadvantages of prior art approaches.
The past approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not to be considered prior art to the claims in this application merely due to the presence of these approaches in this background section.